TECHNICAL FIELD
[0001] The present invention relates to a method for producing an anion-modified cellulose
nanofiber dispersion.
BACKGROUND ART
[0002] When a starting cellulosic material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidin-N-oxyradical
(hereinafter "TEMPO") and sodium hypochlorite which is an inexpensive oxidizing agent,
carboxyl groups can be introduced efficiently onto the cellulose microfibril surface
(Non-patent Document 1, Patent Documents 1 and 2). Further, carboxymethyl groups can
be introduced into cellulose by mercerization of a starting cellulosic material and
subsequent reaction with monochloroacetic acid or sodium monochloroacetate (Patent
Documents 3 and 4). In the cellulose into which carboxyl groups (-COOH) or carboxymethyl
groups (-CH2COOH) have been introduced, such groups are negatively charged in a solvent
(hereinafter, the cellulose is referred to as "anion-modified cellulose"). It is known
that when such anion-modified cellulose is treated in a solvent with a mixer or the
like, a dispersion of cellulose nanofibers, which are cellulose microfibrils, is obtained
(Non-patent Document 1, Patent Documents 1, 2 and 4).
[0003] Cellulose nanofibers are a water-dispersible and biodegradable material. Since cellulose
nanofibers obtained by the method mentioned above are in the form of a dispersion,
they can be blended with various water-soluble polymers or can form a composite with
an organic or inorganic pigment and be modified consequently. Further, the cellulose
nanofibers can be formed into a sheet or fiber. These characteristics of cellulose
nanofibers are expected to be of help in developing novel and highly functional products
by application of the cellulose nanofibers to highly functional packaging materials,
transparent organic substrate materials, highly functional fibers, separation membranes,
regenerative biomaterials and the like.
CITATION LIST
PATENT DOCUMENTS
NON-PATENT DOCUMENT
SUMMARY
TECHNICAL PROBLEM
[0006] In consideration of application to the above-mentioned uses, it is preferred to produce
a high concentration of cellulose nanofiber dispersion. However, there is the problem
that a high concentration of such a dispersion has high viscosity, which makes the
dispersion difficult to handle.
[0007] The present invention aims to provide a method for producing a cellulose nanofiber
dispersion by which such a dispersion having superior fluidity can be produced.
SOLUTION TO PROBLEM
[0008] As a result of extensive and intensive studies, the present inventors found that
addition of a certain amount of a monovalent or divalent metal ion to an anion-modified
cellulose nanofiber dispersion can increase the fluidity of the dispersion. The monovalent
or divalent metal ion may be added as defined in the claims, and it is preferred that
the amount of the monovalent or divalent metal ion contained in a final dispersion
is 0.5 to 10% (w/w) based on the amount of anion-modified cellulose. The monovalent
or divalent metal ion is a metal ion contained in the liquid portion of the reaction
mixture generated from the under-mentioned alkaline hydrolysis of anion-modified cellulose.
More specifically, the liquid obtained by solid-liquid separation of the reaction
mixture generated from the alkaline hydrolysis of anion-modified cellulose may be
added to a dispersion to incorporate a predetermined concentration of a monovalent
or divalent metal ion into a final dispersion. Further, in the reaction mixture generated
from the alkaline hydrolysis of anion-modified cellulose, the cellulose may be defibrated,
without being washed, to form it into nanofibers to prepare an anion-modified cellulose
nanofiber dispersion. By this process, the monovalent or divalent metal ion that remains
in the reaction mixture generated from the alkaline hydrolysis is incorporated into
the dispersion that, in consequence, has superior fluidity.
[0009] More specifically, the present invention provides the following methods [1] to [4]:
- [1] A method for producing an anion-modified cellulose nanofiber dispersion, comprising
the steps of:
- (A) preparing anion-modified cellulose; and
- (B) defibrating the anion-modified cellulose into nanofibers to prepare the anion-modified
cellulose nanofiber dispersion,
the method further comprisingthe step of adding a compound containing a monovalent
or divalent metal ion, wherein an amount of the added metal ion is 0.3 to 10% (w/w)
based on anion-modified cellulose nanofibers contained in the final anion-modified
cellulose nanofiber dispersion;
characterised in that the monovalent or divalent metal ion is a metal ion contained
in a liquid portion of the reaction mixture generated from alkaline hydrolysis of
a second anion-modified cellulose and wherein either:
- (i) the second anion-modified cellulose is the anion-modified cellulose of step A
and wherein the method comprises subjecting the reaction mixture from the alkaline
hydrolysis to solid-liquid separation to recover the liquid portion, and adding the
liquid portion to the cellulose nanofiber dispersion obtained from step B; or
- (ii) the second anion-modified cellulose is not the anion-modified cellulose of step
A and wherein the method comprises subjecting the reaction mixture from the alkaline
hydrolysis to solid-liquid separation to recover the liquid-portion, and adding the
liquid portion to the cellulose nanofiber dispersion obtained from step B without
the alkaline hydrolysis treatment; or
- (iii) the second anion-modified cellulose is the anion-modified cellulose of step
A and wherein step B comprises defibrating the anion-modified cellulose contained
in the reaction mixture from the alkaline hydrolysis into nanofibers, wherein the
anion-modified cellulose obtained after completion of the alkaline hydrolysis is defibrated
without being washed.
- [2] The method according to [1], wherein the step of preparing anion-modified cellulose
comprises introducing carboxymethyl groups into cellulose and a degree of substitution
of carboxymethyl group per glucose unit of the anion-modified cellulose is 0.01 to
0.50.
- [3] The method according to [1], wherein the step of preparing anion-modified cellulose
comprises introducing carboxyl groups into cellulose and an amount of the introduced
carboxyl groups is 1.0 to 2.0 mmol/g based on a bone dry mass of the anion-modified
cellulose.
- [4] The method according to any one of [1] to [3], wherein:
step A comprises oxidizing a starting cellulosic material with an oxidizing agent
in the presence of an N-oxyl compound and a compound selected from the group consisting
of bromides, iodides and mixtures thereof to introduce carboxyl groups into cellulose
to prepare anion-modified cellulose; and
wherein the step of subjecting the anion-modified cellulose to alkaline hydrolysis
uses an oxidizing agent or a reducing agent under alkaline conditions.
ADVANTAGEOUS EFFECTS OF INVENTION
[0010] In accordance with the present invention, a cellulose nanofiber dispersion having
superior fluidity can be produced.
DESCRIPTION OF EMBODIMENTS
[0011] The present invention is described in detail below. As referred to herein, values
of "A to B" mean values ranging from A to B, both ends inclusive.
1. Anion-modified cellulose nanofibers
[0012] The "anion-modified cellulose nanofibers" as referred to in the present invention
mean cellulose microfibrils having an average fiber length of 50 to 5000 nm, preferably
0.1 to 5 µm and an average fiber width of 1 to 1000 nm or 2 to 300 nm, preferably
2 to 150 nm that are obtainable by defibration of a starting cellulosic material into
which anionic groups (e.g., carboxyl groups or carboxymethyl groups) have been introduced
("anion-modified cellulose"). If the anion-modified cellulose is cellulose into which
carboxyl groups have been introduced, cellulose nanofibers obtained are preferably
single cellulose microfibrils having a width of about 2 to 5 nm and a length of about
1 to 5 µm or cellulose nanofibers having a width of about 2 to 300 nm and a length
of about 100 to 500 nm.
< Starting cellulosic material >
[0013] The starting cellulosic material that is used encompasses, for example, kraft pulp
or sulfite pulp of wood origin; cellulose powder formed by pulverizing such pulp with
a highpressure homogenizer, a mill or the like; and a microcrystalline cellulose powder
formed by purifying such a material by chemical treatment such as acid hydrolysis.
Other starting cellulosic materials may be derived from plant such as kenaf, hemp,
rice, bagasse or bamboo. It is preferred to use cellulose powder, microcrystalline
cellulose powder, or chemical pulp such as kraft pulp or sulfite pulp, in terms of
mass production and cost. Use of cellulose powder or microcrystalline cellulose powder
provides cellulose nanofibers that form dispersion having low viscosity even in high
concentration. When the chemical pulp is used, it is preferable to subject the pulp
to a known bleaching treatment to remove lignin. The bleached pulp that can be used
may be, for example, bleached kraft pulp or bleached sulfite pulp which has a brightness
(ISO 2470) of 80% or more.
[0014] The "cellulose powder" means a rod-like particle consisting of microcrystalline or
crystalline cellulose which is obtained by an acid hydrolysis to remove amorphous
region from wood pulp, followed by pulverizing and sieving. In the cellulose powder,
the degree of polymerization of cellulose is about 100 to 500, the crystallinity of
the cellulose powder as measured by X-ray diffraction is 70 to 90%, and the volume
average particle size as measured with an apparatus for laser diffraction particle
size distribution is preferably 100 µm or less, more preferably 50 µm or less. When
the volume average particle size is 100 µm or less, a cellulose nanofiber dispersion
having more fluidity can be obtained. Such a cellulose powder may be prepared by subjecting
well-selected pulp to acid hydrolysis to obtain an undecomposed residue and purifying,
drying, pulverizing and sieving the residue, or may be a commercially available product
such as KC FLOCK
R (Nippon Paper Chemicals Co., Ltd.), CEOLUS
R (Asahi Kasei Chemicals Corporation), or AVICEL
R (FMC Corporation).
[0015] Further, a material prepared by making the starting cellulosic material fine with
a dispersing apparatus such as a high-speed rotary apparatus, a colloid mill apparatus,
a high pressure apparatus, a roll mill apparatus, or an ultrasound apparatus, a wet
high pressure or ultrahigh pressure homogenizer, or the like also may be used as a
starting cellulosic material.
< Anion-modified cellulose >
[0016] Anion-modified cellulose can be obtained by introducing anionic groups into the starting
cellulosic material by the known methods exemplified below (anionic modification).
(1-1) Carboxymethylation
[0017] The starting cellulosic material is used as a starting material, and a solvent for
this process is a mixed medium of water and a lower alcohol that is used with a mass
3 to 20 times the mass of the starting material. The lower alcohol includes methanol,
ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary butanol
or the like, or combinations thereof. The mixing proportion of the lower alcohol in
the mixed medium is 60 to 95 mass %. An alkali metal hydroxide, more specifically,
sodium hydroxide or potassium hydroxide, is used as a mercerizing agent in an amount
of 0.5 to 20 moles per mole of glucose residue of the starting material. The starting
material, the solvent, and the mercerizing agent are mixed and a mercerization treatment
is carried out at a reaction temperature of 0 to 70°C, preferably 10 to 60°C, for
a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours. After
that, monochloroacetic acid or sodium monochloroacetate (a carboxymethylating agent)
is added in an amount of 0.05 to 10.0 moles per mole of glucose residue, and an etherification
reaction is carried out at a reaction temperature of 30 to 90°C, preferably 40 to
80°C, for a reaction time of 30 minutes to 10 hours, preferably 1 to 4 hours, by which
carboxymethyl group-introduced cellulose can be obtained.
[0018] If the carboxymethyl group-introduced cellulose is used as anion-modified cellulose,
the degree of substitution of carboxymethyl group per glucose unit of the cellulose
is preferably 0.01 to 0.50. Introduction of carboxymethyl groups into celluloses cause
them to repel each other electrically. Thus, carboxymethyl group-introduced cellulose
can be defibrated easily into fibers that are on the order of nanometers. When the
degree of substitution of carboxymethyl groupper glucose unit is less than 0.01, the
cellulose cannot be defibrated sufficiently. In contrast, when the degree of substitution
of carboxymethyl group per glucose unit is more than 0.50, the cellulose is swollen
or dissolved during defibration and nanofibers may not be obtained.
[0019] The degree of substitution of carboxymethyl group can be determined by the following
method:
About 2.0 g of a sample is weighed precisely and put into a 300 ml stoppered Erlenmeyer
flask. 100 ml of nitric acid-methanol (a solution of 100 ml special grade concentrated
nitric acid in 1 L anhydrous methanol) is added and the mixture is shaken for 3 hours
to convert sodium carboxymethylcellulose (Na-CMC) into carboxymethylcellulose (H-CMC).
1.5 to 2.0 g of bone-dry H-CMC is weighed precisely and put into a 300 ml stoppered
Erlenmeyer flask. The H-CMC is wetted with 15 ml of 80% methanol, 100 ml of 0.1N NaOH
is added, and the mixture is shaken at room temperature for 3 hours. An excess of
NaOH is back-titrated with 0.1N H
2SO
4 using phenolphthalein as an indicator. The degree of substitution of carboxymethyl
group is calculated using the following formulas:
A: Amount (ml) of IN NaOH required for neutralization of 1 g of H-CMC
F': Factor for 0.1N H2SO4
F: Factor for 0.1 N NaOH
(1-2) Carboxylation
[0020] Carboxyl group-introduced cellulose (hereinafter also referred to as "oxidized cellulose")
can be obtained by oxidation of the starting cellulosic material in water using an
oxidizing agent in the presence of an N-oxyl compound and a compound selected from
the group consisting of bromides, iodides and mixtures thereof.
[0021] The "N-oxyl compound" means a compound that can generate a nitroxyl radical. The
N-oxyl compound may be any compound as long as it promotes desired oxidation reaction.
Examples include compounds represented by the following general formula (Formula 1):
wherein R
1 to R
4, which are the same or different, each represent an alkyl group having about 1 to
4 carbon atoms.
[0022] Among the substances represented by Formula 1, 2,2,6,6-tetramethyl-1-piperidinoxyradical
(hereinafter "TEMPO") is preferred. Further, N-oxyl compounds represented by any one
of Formulas 2 to 5 below, i.e., 4-hydroxy TEMPO derivatives to which appropriate hydrophobicity
has been imparted by etherification of the hydroxyl group of 4-hydroxy TEMPO with
an alcohol or esterification with carboxylic acid or sulfonic acid, or 4-acetamide
TEMPO to which appropriate hydrophobicity has been imparted by acetylation of the
amino group of 4-amino TEMPO, are preferred since they are inexpensive and enable
homogeneous oxidized cellulose to be obtained.
In Formulas 2 to 4, R is a straight or branched carbon chain having 4 or less carbon
atoms.
[0023] Further, N-oxyl compounds represented by Formula 6 shown below, i.e., azaadamantane
type nitroxyl radicals, are preferred since they can oxidize a starting cellulosic
material efficiently within a short reaction time and tend not to cause cellulose
chains to be broken.
wherein R
5 and R
6, which are the same or different, each represent a hydrogen atom or a C
1-C
6 straight or branched alkyl group.
[0024] The amount of an N-oxyl compound used is not particularly limited as long as it is
a catalytic amount that enables defibration of cellulose into nanofibers. For example,
the amount is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, even more
preferably 0.05 to 0.5 mmol, based on 1 g (bone dry weight) of a starting cellulosic
material.
[0025] The "bromide" means a bromine-containing compound and examples include alkali metal
bromides which can be dissociated in water and ionized. The "iodide" means an iodine-containing
compound and examples include alkali metal iodides. The amount of the bromide or iodide
used may be selected within a range of amount that can promote oxidation reaction.
For example, the total amount of the bromide or the iodide is preferably 0.1 to 100
mmol, more preferably 0.1 to 10 mmol, even more preferably 0.5 to 5 mmol, based on
1 g (bone dry weight) of a starting cellulosic material.
[0026] The oxidizing agent may be a known oxidizing agent, for example, a halogen, a hypohalogenous
acid, a halogenous acid, a perhalogenic acid, or a salt thereof, a halogen oxide,
or a peroxide. Among them, sodium hypochlorite, which is inexpensive and less harmful
to the environment, is preferred. The appropriate amount of an oxidizing agent used
is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even
more preferably 1 to 25 mmol, most preferably 3 to 10 mmol, based on 1 g (bone dry
weight) of a starting cellulosic material.
[0027] In the step of cellulose oxidation, it is possible to proceed with reaction efficiently
even under relatively mild conditions. Hence, the reaction temperature may be a room
temperature of about 15 to 30°C. As the reaction proceeds, carboxyl groups are generated
in cellulose and hence, a decline in the pH of reaction mixture is observed. To proceed
with the oxidation reaction efficiently, it is preferable to maintain the pH of reaction
mixture at about pH 9 to 12, preferably about pH 10 to 11, by adding an alkaline solution
such as an aqueous sodium hydroxide solution. The reaction medium is preferably water
in terms of handleability, less tendency to cause a side reaction, and the like.
[0028] The reaction time in the oxidation reaction may be set as appropriate according to
the degree of progression of oxidation and is generally about 0.5 to 6 hours, for
example, about 0.5 to 4 hours.
[0029] The oxidation reaction may be carried out in two stages. For example, oxidized cellulose
that is obtained by separation through filtration after completion of reaction in
the first stage is oxidized again under the same or different reaction conditions,
and this oxidation enables efficient introduction of carboxyl groups into a starting
cellulosic material without reaction inhibition due to sodium chloride produced as
a by-product of the reaction in the first stage.
[0030] It is preferable to set conditions such that the amount of carboxyl groups contained
in the oxidized cellulose is 0.2 to 2.0 mmol/g or more based on the bone dry mass
of the cellulose. The amount is more preferably 1.0 to 2.0 mmol/g. The amount of the
carboxyl groups can be adjusted by a means such as adjustment of the oxidation reaction
time, the oxidation reaction temperature, the pH during the oxidation reaction, or
the additive amount of the N-oxyl compound, bromide, iodide, or oxidizing agent.
[0031] The obtained oxidized cellulose is preferably washed.
[0032] The amount of the carboxyl groups can be calculated through the following steps:
preparing 60 ml of 0.5 mass % slurry (aqueous dispersion) of the oxidized cellulose
or cellulose nanofibers, adjusting the slurry to pH 2.5 with a 0.1M aqueous hydrochloric
acid solution, then adding a 0.05N aqueous sodium hydroxide solution dropwise while
measuring the electrical conductivity until the pH reaches 11, and calculating the
amount of carboxyl groups on the basis of the amount of sodium hydroxide (a) consumed
in the stage of neutralization with weak acid where the electrical conductivity changes
slowly, using the following formula:
< Defibration of anion-modified cellulose into nanofibers >
[0033] A dispersion containing the anion-modified cellulose that is obtained as described
above is prepared and the cellulose is defibrated in the dispersion to form nanofibers.
"To defibrate into nanofibers" means processing the cellulose into cellulose fibers
that have an average fiber width of 1 to 1000 nm or 2 to 300 nm, preferably 2 to 150
nm, and an average fiber length of 50 to 5000 nm, preferably 0.1 to 5 µm. If the anion-modified
cellulose is cellulose into which carboxyl groups have been introduced, cellulose
fibers obtained preferably have a width of about 2 to 5 nm and a length of about 1
to 5 µm, or a width of about 2 to 300 nm and a length of about 100 to 500 nm. In the
"dispersion," the anion-modified cellulose is dispersed in a dispersion medium. The
dispersion medium is preferably water in terms of handleability.
[0034] To defibrate the anion-modified cellulose and disperse it in a dispersion medium,
it is preferable to apply strong shear force to the dispersion with an apparatus such
as a high-speed rotary apparatus, a colloid mill apparatus, a high pressure apparatus,
a roll mill apparatus, or an ultrasound apparatus. To obtain anion-modified cellulose
nanofibers efficiently, it is particularly preferable to use a wet high pressure or
ultrahigh pressure homogenizer that can apply to the dispersion a pressure of 50 MPa
or higher and strong shear force. The above-mentioned pressure is more preferably
100 MPa or higher, even more preferably 140 MPa or higher. By this treatment, anion-modified
cellulose is defibrated and anion-modified cellulose nanofibers are formed and dispersed
in a dispersion medium.
[0035] The concentration of anion-modified cellulose in a dispersion subjected to the above-mentioned
treatment is 0.1% (w/v) or higher, preferably 1 to 50% (w/v), more preferably 1 to
10% (w/v), or may be 2 to 10% (w/v) or 3 to 10% (w/v).
2. Compound containing monovalent or divalent metal ion
[0036] The present invention increases the fluidity of anion-modified cellulose nanofiber
dispersion by addition of a monovalent or divalent metal ion to the dispersion. The
monovalent or divalent metal ion is added as defined in the claims. For example, the
ion may be added after production of anion-modified cellulose, or after defibration
of anion-modified cellulose into nanofibers to prepare an anion-modified cellulose
nanofiber dispersion. In terms of fluidity increase, the amount of the added monovalent
or divalent metal ion in a final anion-modified cellulose nanofiber dispersion is
0.3 to 10% (w/w) based on the amount of anion-modified cellulose (including anion-modified
cellulose nanofibers). The amount of the added metal ion is more preferably 0.4 to
10% (w/w), even more preferably 0.5 to 8% (w/w). Even if increasing the amount of
the added metal ion to more than 10% (w/w), there is no further increase in viscosity-reducing
effect. In contrast, when the amount is lower than 0.3% (w/w), sufficient effect cannot
be obtained. As referred to herein, the "amount of added metal ion" does not take
into consideration a metal ion combined with anion-modified cellulose (for example,
Na ion carried with carboxyl group (-COONa) in carboxyl group-introduced cellulose).
The expression "an amount of added metal ion in a final anion-modified cellulose nanofiber
dispersion" is intended to mean that a metal ion that is not included in a final dispersion
is not taken into consideration; for example, even if a certain amount of metal ion
is added during production of anion-modified cellulose nanofiber dispersion, a part
of the added metal ion that is removed from the dispersion by washing or the like
and is not included in the final dispersion, is not taken into consideration.
[0037] When a known concentration of a metal ion is added in the production process, the
amount of the added metal ion in an anion-modified cellulose nanofiber dispersion
can be calculated. Further, the amount of the added metal ion in a final dispersion
can be determined by a known method for measuring the concentration of metal ion observed
at each stage of production. To measure the concentration of metal ion, for example,
a commercially available ion concentration meter (e.g., IM-32P, a portable ion meter
produced by DKK-TOA Corporation) may be used.
[0038] Examples of the monovalent or divalent metal ion include ions such as sodium ion,
magnesium ion, potassium ion, and calcium ion. Sodium ion or potassium ion is preferred.
The monovalent or divalent metal ion may be added in the form of any inorganic salt
or organic salt. Examples of such a compound containing a monovalent or divalent metal
ion include the following salts of the metal ions: chloride, bromide, iodide, hydroxide,
nitrate, nitrite, dihydrogenphosphate, sulfate, sulfite, thiosulfate, carbonate, phosphate,
acetate, pyrophosphate, polyphosphate, tripolyphosphate, tetrapolyphosphate, metaphosphate,
trimetaphosphate, tetrametaphosphate, hexametaphosphate, polyacrylate, polymethacrylate,
polyitaconate, orthosilicate, metasilicate, phosphonate, polymaleic acid copolymer
salt, humate, tannate, dodecylsulfate, dodecylbenzene sulfonate, polystyrene sulfonate,
and lignin sulfonate; and polyesters having sulfonic acid group(s) and/or salts thereof.
In the case of the organic acid salts, the organic acid is not limited to a homopolymer
but may be a copolymer. For example, the organic acid may be a copolymer of polyacrylic
acid or polymethacrylic acid with another monomer. Examples of the monomer for the
copolymerization include unsaturated carboxylic acids such as α-hydroxyacrylic acid,
crotonic acid, maleic acid, itaconic acid and fumaric acid, and unsaturated sulfonic
acids such as 2-acrylamide-2-methylpropanesulfonic acid, (meth)arylsulfonic acid,
and styrenesulfonic acid. The compound containing a monovalent or divalent metal ion
may be a mixture of the above components.
[0039] Alternatively, a waste liquor obtained from the reaction mixture generated from the
under-mentioned alkaline hydrolysis of anion-modified cellulose may be used as the
compound containing a monovalent or divalent metal ion.
[0040] While not wishing to be bound by any particular theory, the present inventors infer
the reason for advantageous effects to be exhibited in the present invention as follows:
the anionic groups localized on the surface of anion-modified cellulose nanofibers
form a hydrated layer around the cellulose nanofibers, and the cellulose nanofibers
are present close to each other to form a network; when a certain amount of a compound
containing a monovalent or divalent metal ion is contained in the anion-modified cellulose
nanofibers, the cellulose nanofibers partially bind to each other and, as a result,
the number of particles contained in a dispersion decreases and the viscosity of the
dispersion is reduced.
3. Alkaline hydrolysis
[0041] When carboxyl group-introduced cellulose is used as anion-modified cellulose, hydrolysis
may be carried out arbitrarily under alkaline conditions ("alkaline hydrolysis") before
production of nanofibers. The alkaline hydrolysis may be carried out using water as
a reaction medium. Further, an oxidizing agent or a reducing agent is used as an auxiliary
agent. The oxidizing agent or reducing agent may be one that has activity in an alkaline
range. The oxidizing agent includes oxygen, ozone, hydrogen peroxide and hypochlorite,
or combinations thereof. The reducing agent includes sodium boron hydride, hydrosulfite
and sulfite, or combinations thereof. In terms of reaction efficiency, the additive
amount of the auxiliary agent is preferably 0.1 to 10 mass %, more preferably 0.3
to 5 mass %, even more preferably 0.5 to 2 mass %, based on bone-dry anion-modified
cellulose.
[0042] The pH of the system in the alkaline hydrolysis reaction is preferably 8 to 14, more
preferably 9 to 13, even more preferably 10 to 12. The alkali that is used may be
any water-soluble alkali, but sodium hydroxide is best in terms of production cost.
The temperature is preferably 40 to 120°C, more preferably 50 to 100°C, even more
preferably 60 to 90°C. The reaction time is preferably 0.5 to 24 hours, more preferably
1 to 10 hours, even more preferably 2 to 6 hours. The concentration of anion-modified
cellulose in the system is preferably 1 to 20 mass %, more preferably 3 to 15 mass
%, even more preferably 5 to 10 mass %.
[0043] The alkaline hydrolysis can reduce the energy required for defibration of anion-modified
cellulose into nanofibers. The reason for this reduction is not clear but is inferred
as follows:
Carboxyl groups are located in the amorphous region of cellulose oxidized with an
N-oxyl compound, and the hydrogen at the C6-position at which a carboxyl group exists
lacks electric charge since the electron of the hydrogen atom has been attracted by
the carboxyl group; hence, under alkaline conditions, the hydrogen is easily removed
by a hydroxide ion and, as a result, cleavage reaction of glycoside bonds due to β-elimination
proceeds and fiber length of the oxidized cellulose becomes shorter. By allowing the
fiber length of oxidized cellulose fibers to be shortened in this way, the viscosity
of a dispersion containing the starting material can be lowered. In consequence, the
energy required for the subsequent step of production of nanofibers is reduced. It
should be noted that when the cellulose is merely hydrolyzed under alkaline conditions,
the cellulose is likely to take on a yellow color. It is presumed that this is due
to generation of double bonds during β-elimination. If an oxidizing agent or a reducing
agent is used in the hydrolysis under alkaline conditions, the double bonds can be
oxidized or reduced to be removed, and hence, coloration is suppressed.
< Use of waste liquor obtained from alkaline hydrolysis reaction >
(3-1) Addition of waste liquor obtained from alkaline hydrolysis reaction
[0044] The present invention increases the fluidity of dispersion by addition of a compound
containing a monovalent or divalent metal ion to the dispersion, and such a compound
may be waste liquor obtained from the above-mentioned alkaline hydrolysis reaction.
In this case, the reaction mixture obtained after completion of the alkaline hydrolysis
reaction is subjected to solid-liquid separation and the liquid obtained is used as
the "waste liquor". Examples of a solid-liquid separator that is used include, but
are not particularly limited to, vertical or horizontal centrifugal solid-liquid separation
and dewatering apparatuses such as a decanter type centrifuge separator; filter-type
solid-liquid separation and dewatering apparatuses such as a vacuum filter, a pressure
filter, and a drum displacer; and press-type solid-liquid separation and dewatering
apparatuses such as a filter press, a belt press, a screw press, a twin roll press,
and a wash press.
[0045] The waste liquor obtained from the alkaline hydrolysis reaction may be added to a
cellulose nanofiber dispersion obtained by defibration of anion-modified cellulose
with the alkaline hydrolysis treatment, or to a cellulose nanofiber dispersion obtained
by defibration of anion-modified cellulose without the alkaline hydrolysis treatment.
The additive amount of the waste liquor may be determined according to the concentration
of the monovalent or divalent metal ion contained in the waste liquor.
(3-2) Defibration of unwashed cellulose after alkaline hydrolysis reaction
[0046] One of methods for use of the waste liquor obtained from the alkaline hydrolysis
may be a method in which cellulose obtained after completion of the alkaline hydrolysis
reaction is defibrated without being washed, which allows a cellulose nanofiber dispersion
to contain the monovalent or divalent metal ion contained in the reaction mixture
generated from the alkaline hydrolysis. Alkaline hydrolysis of anion-modified cellulose
(carboxyl group-introduced cellulose) and subsequent defibration of the unwashed cellulose,
as described above, allow a cellulose nanofiber dispersion to contain the monovalent
or divalent metal ion that remains in the reaction mixture generated from the alkaline
hydrolysis.
[0047] The cellulose nanofibers obtained by alkaline hydrolysis of carboxyl group-introduced
cellulose and defibration of the unwashed cellulose are single cellulose microfibrils
having a width of about 2 to 300 nm and a length of about 100 to 500 nm, or a width
of about 2 to 5 nm and a length of about 1 to 5 µm. As to the cellulose nanofibers,
the B-type viscosity (60 rpm, 20°C) in an aqueous dispersion having a concentration
of 5.0% (w/v) is 3000 mPa•s or less, preferably 2000 mPa•s or less, more preferably
1000 mPa•s or less. When the B-type viscosity in an aqueous dispersion having a concentration
of 5% (w/v) is 3000 mPa•s or less, superior miscibility with various pigments, binder
resins and the like is obtained. Further, when the B-type viscosity is 1000 Pa•s or
less, a coating layer having at least a certain film thickness and superior surface
nature can be obtained efficiently. The lower limit of the B-type viscosity is not
particularly limited but is generally at least about 1 mPa•s or at least about 5 mPa•s.
The B-type viscosity may be measured with a common B-type viscometer, and for example,
TV-10 viscometer, which is a product of Toki Sangyo Co., Ltd., may be used to measure
the B-type viscosity under the conditions of 20°C and 60 rpm. Since the cellulose
nanofibers are superior in fluidity as well as barrier property and heat resistance,
they can be used for various applications such as packaging materials.
4. Viscosity-reducing treatment
[0048] When carboxyl group-introduced cellulose is used as anion-modified cellulose, a method
different from the above-mentioned alkaline hydrolysis may be used arbitrarily to
subject the cellulose to a viscosity-reducing treatment before defibration of the
cellulose into nanofibers. This treatment can reduce the energy required for the production
of nanofibers. The "viscosity-reducing treatment" means a treatment of cutting anion-modified
cellulose chains into pieces of appropriate length (production of the cellulose chains
into short fibers). Given that dispersion prepared using the thus treated starting
material has low viscosity, the viscosity-reducing treatment can also be regarded
as a treatment for obtaining anion-modified cellulose that forms low-viscosity dispersion.
The viscosity-reducing treatment may be a treatment that reduces the viscosity of
anion-modified cellulose, and it is, for example, a treatment of irradiating anion-modified
cellulose with ultraviolet rays, a treatment of oxidizing and decomposing anion-modified
cellulose using hydrogen peroxide and ozone, a treatment of hydrolyzing anion-modified
cellulose with an acid, or a combination of these treatments. Such a viscosity-reducing
treatment may be carried out before alkaline hydrolysis or between alkaline hydrolysis
and production of nanofibers. Alternatively, before production of nanofibers, only
the viscosity-reducing treatment may be carried out without alkaline hydrolysis.
(4-1) Ultraviolet irradiation
[0049] When a viscosity-reducing treatment is carried out by irradiation of anion-modified
cellulose with ultraviolet rays, the wavelength of the ultraviolet rays is preferably
100 to 400 nm, more preferably 100 to 300 nm. Especially, ultraviolet rays of wavelength
135 to 260 nm are preferred since they can act directly on cellulose or hemicellulose
to reduce its molecular weight and form anion-modified cellulose into short fibers.
[0050] As a light source for the ultraviolet irradiation, one that can irradiate light in
a wavelength range of 100 to 400 nm may be used. Specific examples include a xenon
short arc lamp, an ultrahigh pressure mercury lamp, a high pressure mercury lamp,
a low pressure mercury lamp, a deuterium lamp, and a metal halide lamp. These light
sources may be used singly or in combination of any two or more. In particular, when
a combination of a plurality of light sources having different wavelength characteristics
is used, ultraviolet rays of different wavelengths can be irradiated simultaneously,
thereby increasing dissociation sites of cellulose chains or hemicellulose chains;
hence, production of short fibers can be promoted.
[0051] As a vessel in which anion-modified cellulose is placed, for example, a vessel made
of hard glass may be used when ultraviolet rays of a wavelength longer than 300 nm
are used. On the other hand, when ultraviolet rays of a wavelength shorter than 300
nm are used, it is preferable to use a vessel made of quartz glass which transmits
more ultraviolet rays. In the vessel, a material of the part that is not involved
in the reaction with ultraviolet rays may be selected as appropriate from materials
less likely to be deteriorated for the wavelength of ultraviolet rays.
[0052] To proceed with the reaction efficiently, it is preferable to prepare a dispersion
by dispersing anion-modified cellulose in a dispersion medium and irradiate the dispersion
with ultraviolet rays. The dispersion medium is preferably water, from the viewpoints
of suppression of any side reaction and the like. The concentration of anion-modified
cellulose in the dispersion is preferably 0.1 mass % or higher, from the viewpoint
of enhancement of energy efficiency. Further, the concentration is preferably 12 mass
% or lower to maintain good fluidity of anion-modified cellulose in an ultraviolet
irradiator and enhance reaction efficiency. Hence, the concentration of anion-modified
cellulose in the dispersion is preferably 0.1 to 12 mass %, more preferably 0.5 to
5 mass %, even more preferably 1 to 3 mass %.
[0053] The reaction temperature is preferably 20°C or higher in terms of reaction efficiency.
Meanwhile, when the temperature is too high, anion-modified cellulose can be deteriorated
or the pressure in a reactor apparatus can exceed atmospheric pressure; hence, the
reaction temperature is preferably 95°C or lower. Accordingly, the reaction temperature
is preferably 20 to 95°C, more preferably 20 to 80°C, even more preferably 20 to 50°C.
Further, when the reaction temperature falls within the range, there is the advantage
that it is unnecessary to take pressure resistance into consideration in apparatus
designing. The pH of the system in the reaction is not limited, but the neutral range,
for example, about pH 6.0 to 8.0, is preferred in light of simplification of process.
[0054] The level of the ultraviolet irradiation may be set arbitrarily by a means such as
adjusting the residence time of anion-modified cellulose in an irradiation reactor
or adjusting the amount of energy of an irradiation light source. For example, the
amount of ultraviolet rays with which anion-modified cellulose is irradiated may be
adjusted by diluting with water or the like the dispersion of anion-modified cellulose
in an irradiation apparatus or diluting the dispersion by blowing air or an inert
gas such as nitrogen into the dispersion. These conditions are selected as appropriate
to obtain desired values of quality of the treated starting material (e.g., fiber
length, polymerization degree of cellulose).
[0055] It is preferable to perform the ultraviolet irradiation in the presence of an auxiliary
agent such as oxygen, ozone or a peroxide (e.g., hydrogen peroxide, peracetic acid,
sodium percarbonate, sodium perborate), since the efficiency of the photo-oxidation
reaction can be enhanced. Especially, in the case of irradiation with ultraviolet
rays in a wavelength range of 135 to 242 nm, it is preferred that the ozone generated
from air which is present generally in a vapor phase area near a light source is used
as an auxiliary agent. For example, while air is continuously supplied to an area
near a light source, ozone generated is continuously withdrawn and injected into anion-modified
cellulose; in consequence, ozone can be utilized as an auxiliary agent in the photo-oxidation
reaction without supplying ozone from the outside of the system. Moreover, by supplying
oxygen to the vapor phase area near a light source, more ozone can be generated in
the system. In this manner, ozone generated secondarily in an ultraviolet irradiation
reactor can be utilized.
[0056] The ultraviolet irradiation may be repeated plural times. The number of the repetitions
may be set as appropriate according to the quality of the treated starting material,
the relation with post-treatment such as bleaching, and the like. For example, ultraviolet
rays of 100 to 400 nm, preferably 135 to 260 nm, may be irradiated about 1 to 10 times,
preferably about 2 to 5 times. In this case, the irradiation time is preferably 0.5
to 10 hours each time, more preferably 0.5 to 3 hours each time.
(4-2) Oxidative decomposition using hydrogen peroxide and ozone
[0057] Ozone may be generated in an ozone generator by a known method using air or oxygen
as a starting material. As described above, to perform an oxidation reaction efficiently,
it is preferred to use a dispersion prepared by dispersing anion-modified cellulose
in a dispersion medium such as water. The amount (mass) of ozone that is used is preferably
0.1 to 3 times larger than the bone dry mass of anion-modified cellulose. When the
amount of ozone is at least 0.1 time larger than the bone dry mass of anion-modified
cellulose, the amorphous region of the cellulose can be decomposed sufficiently, which
enables reduction in the energy required for the subsequent step of production of
nanofibers. Meanwhile, when the amount of ozone is excessively large, cellulose can
be decomposed excessively, but when the amount of use is not more than 3 times larger
than the bone dry mass of anion-modified cellulose, excessive decomposition can be
suppressed. The amount of ozone is more preferably 0.3 to 2.5 times, even more preferably
0.5 to 1.5 times larger than the bone dry mass of anion-modified cellulose.
[0058] The amount (mass) of hydrogen peroxide that is used is preferably 0.001 to 1.5 times
larger than the bone dry mass of anion-modified cellulose. When hydrogen peroxide
is used in an amount that is at least 0.001 time larger than the bone dry mass of
anion-modified cellulose, synergistic action of ozone and hydrogen peroxide is exerted,
making an efficient reaction possible. Meanwhile, for the decomposition of anion-modified
cellulose, it is sufficient to use hydrogen peroxide in an amount that is not more
than about 1.5 times larger than the bone dry mass of anion-modified cellulose, and
use of hydrogen peroxide in a larger amount leads to cost increase. The amount of
hydrogen peroxide is more preferably 0.1 to 1.0 time larger than the bone dry mass
of anion-modified cellulose.
[0059] In terms of reaction efficiency, the pH of the system in the oxidative decomposition
treatment with ozone and hydrogen peroxide is preferably 2 to 12, more preferably
4 to 10, even more preferably 6 to 8. The temperature is preferably 10 to 90°C, more
preferably 20 to 70°C, even more preferably 30 to 50°C. The treatment time is preferably
1 to 20 hours, more preferably 2 to 10 hours, even more preferably 3 to 6 hours.
[0060] An apparatus for the treatment with ozone and hydrogen peroxide may be a common apparatus.
Examples of the apparatus include a common reactor having a reaction chamber, a stirrer,
a chemical injector, a heater, and a pH electrode.
[0061] After the treatment with ozone and hydrogen peroxide, residual ozone and hydrogen
peroxide in the aqueous solution also act effectively in the step of production of
nanofibers, thus allowing production of cellulose nanofibers that form lower viscosity
dispersion.
[0062] The reason a viscosity-reducing treatment for anion-modified cellulose can be carried
out using hydrogen peroxide and ozone is inferred as follows: between anion-modified
celluloses, microscopic spaces formed by the action of charge repulsive force between
anionic groups exist, whereas such spaces are not found in common pulp; when anion-modified
cellulose is treated with ozone and hydrogen peroxide, hydroxyl radicals which have
superior oxidizability are generated from the ozone and the hydrogen peroxide, infiltrate
the microscopic spaces and oxidize and decompose chains in the cellulose efficiently,
forming it into short fibers.
(4-3) Hydrolysis with acid
[0063] An acid is added to anion-modified cellulose to hydrolyze its cellulose chains ("acid
hydrolysis"). The acid that is preferably used is a mineral acid such as sulfuric
acid, hydrochloric acid, nitric acid or phosphoric acid. As described above, to perform
the reaction efficiently, it is preferable to use a dispersion prepared by dispersing
anion-modified cellulose in a dispersion medium such as water. The conditions for
the acid hydrolysis may be ones that allow the acid to act on the amorphous region
of the cellulose. For example, the additive amount of the acid is preferably 0.01
to 0.5 mass %, more preferably 0.1 to 0.5 mass %, based on the bone dry mass of anion-modified
cellulose. Addition of the acid in an amount of 0.01 mass % or more is preferred since
hydrolysis of cellulose proceeds and efficiency of production of nanofibers increases.
When the additive amount is 0.5 mass % or less, it is possible to prevent excessive
hydrolysis of cellulose and decrease in the yield of cellulose nanofibers. The pH
of the system during the acid hydrolysis is preferably 2.0 to 4.0, more preferably
at least 2.0 to below 3.0. It is to be noted that when alkali that is used in the
alkaline hydrolysis reaction or the like remains in the dispersion medium for anion-modified
cellulose, it is preferable to adjust the pH of the system to a pH falling within
the range by increasing the additive amount of acid as appropriate. In terms of reaction
efficiency, the reaction is preferably carried out at a temperature of 70 to 120°C
for 1 to 10 hours.
[0064] To perform the step of production of nanofibers efficiently, it is preferable to
perform neutralization with an alkali such as sodium hydroxide after the acid hydrolysis
treatment.
[0065] The reason a viscosity-reducing treatment for anion-modified cellulose can be carried
out by the acid hydrolysis is inferred as follows: as described above, the anionic
groups localized on the surface of anion-modified cellulose form a hydrated layer
around the cellulose, and the celluloses are present close to each other to form a
network; when an acid is added to carry out hydrolysis, the charge balance in the
network is lost, the robust network formed of cellulose molecules is broken up, the
specific surface area of the anion-modified cellulose increases, the production of
short fibers is promoted, and the viscosity of the cellulose is reduced.
EXAMPLES
[0066] The present invention is described in more detail below by means of Examples, but
the invention is not limited thereto.
[Example 1]
[0067] Step A: 500 g (bone dry weight) of unbeaten bleached softwood kraft pulp (brightness:
85%) was added to 500 ml of an aqueous solution in which 780 mg of TEMPO (Sigma Aldrich)
and 75.5 g of sodium bromide were dissolved, and the mixture was stirred until the
pulp was evenly dispersed. To the reaction system, an aqueous sodium hypochlorite
solution was added in an amount of 6.5 mmol/g, and oxidation reaction was initiated.
While the pH of the system decreased during the reaction, a 3M aqueous sodium hydroxide
solution was successively added to adjust the system to pH 10. When sodium hypochlorite
was consumed and the pH of the system did not change any longer, the reaction was
terminated. The mixture obtained from the reaction was passed through a glass filter
to isolate pulp and the pulp was well washed with water to give oxidized pulp (carboxyl
group-introduced pulp). The yield of the pulp was 86% and the time required for the
oxidation reaction was 115 minutes.
[0068] Step B: The oxidized pulp was adjusted to 1.053% (w/v) and treated three times with
an ultrahigh pressure homogenizer (20°C, 140 Mpa) to give an anion-modified cellulose
nanofiber dispersion. A 3% (w/v) aqueous solution of a metal ion-containing compound
(sodium glucuronate) was prepared, 5 ml of the aqueous solution was added to 95 ml
of the cellulose nanofiber dispersion, and the mixture was stirred with homogenizer
at 3000 rpm for 1 minute to give a homogeneous dispersion.
[0069] The B-type viscosity of the obtained dispersion was measured at 60 rpm at 20°C with
TV-10 viscometer, which is a product of Toki Sangyo Co., Ltd.
[0070] Table 1 shows the measurement result of the B-type viscosity and the concentration
(w/w %) of the monovalent or divalent metal ion based on the anion-modified cellulose
contained in the dispersion.
[Example 2]
[0071] A dispersion was obtained in the same manner as in Example 1 except that sodium cellouronate
was used as the metal ion-containing compound in the step B described above.
[Example 3]
[0072] A dispersion was obtained in the same manner as in Example 1 except that a sodium
salt of carboxymethyl cellulose (A02SH, a product of Nippon Paper Chemicals Co., Ltd.)
was used as the metal ion-containing compound in the step B described above.
[Example 4]
[0073] A dispersion was obtained in the same manner as in Example 1 except that sodium ethylenediaminetetraacetate
was used as the metal ion-containing compound in the step B described above.
[Example 5]
[0074] A dispersion was obtained in the same manner as in Example 1 except that sodium phytate
was used as the metal ion-containing compound in the step B described above.
[Example 6]
[0075] A dispersion was obtained in the same manner as in Example 1 except that sodium polyacrylate
(SN-DISPERSANT L-400, a product of San Nopco Ltd.) was used as the metal ion-containing
compound in the step B described above.
[Example 7]
[0076] A dispersion was obtained in the same manner as in Example 1 except that sodium polysulfonate
(A-6012, a product of Kao Corporation) was used as the metal ion-containing compound
in the step B described above.
[Example 8]
[0077] A dispersion was obtained in the same manner as in Example 1 except that sodium polystyrene
sulfonate was used as the metal ion-containing compound in the step B described above.
[Example 9]
[0078] A dispersion was obtained in the same manner as in Example 1 except that a sodium
salt of β-sulfated cyclodextrin was used as the metal ion-containing compound in the
step B described above.
[Example 10]
[0079] A dispersion was obtained in the same manner as in Example 1 except that sodium chloride
was used as the metal ion-containing compound in the step B described above.
[Example 11]
[0080] A dispersion was obtained in the same manner as in Example 1 except that a mixture
of sodium polyacrylate and magnesium polyacrylate (POIZ
R, a product of Kao Corporation) was used as the metal ion-containing compound in the
step B described above.
[Example 12]
[0081] A dispersion was obtained in the same manner as in Example 1 except that magnesium
chloride was used as the metal ion-containing compound in the step B described above.
[Example 13]
[0082] A dispersion was obtained in the same manner as in Example 1 except that potassium
chloride was used as the metal ion-containing compound in the step B described above.
[Example 14]
[0083] Into a stirrer that can stir pulp, 200 g of pulp (LBKP, a product of Nippon Paper
Chemicals Co., Ltd.) and 88 g of sodium hydroxide, which are both on a dry mass basis,
were added, and the mixture was adjusted with water to a pulp solid concentration
of 15%. The mixture was thereafter stirred at 30°C for 30 minutes and then heated
to 70°C, and sodium monochloroacetate was added in an amount of 117 g, based on active
component. After 1-hour reaction, the reaction product was taken out, neutralized
and washed to give anion-modified cellulose having a degree of substitution of carboxymethyl
group of 0.05 per glucose unit (carboxymethyl group-introduced cellulose). The anion-modified
cellulose was thereafter formed into nanofibers in accordance with the step B described
in Example 1 to give an anion-modified cellulose nanofiber dispersion. A 3% (w/v)
aqueous solution of a metal ion-containing compound (sodium chloride) was prepared,
5 ml of the aqueous solution was added to 95 ml of the cellulose nanofiber dispersion
liquid, and the mixture was stirred with a homogenizer at 3000 rpm for 1 minute to
give a homogeneous dispersion. Table 1 shows the measurement result of the B-type
viscosity of the prepared dispersion at 60 rpm at 20°C.
Examples 1-14 are Reference Examples.
[Comparative Example 1]
[0084] From the oxidized pulp that was prepared in the step A of Example 1, a 1.053% (w/v)
cellulose nanofiber dispersion was prepared in accordance with the step B of Example
1. A metal ion-free compound (ultrapure water) was added instead of the metal ion-containing
compound described in the step B of Example 1, and the mixture was stirred with a
homogenizer at 3000 rpm for 1 minute to give a homogeneous dispersion. Table 1 shows
the measurement result of the B-type viscosity of the prepared dispersion at 60 rpm
at 20°C.
[Comparative Example 2]
[0085] A dispersion was prepared in the same manner as in Comparative Example 1 except that
polyvinyl alcohol was used instead of the ultrapure water used in Comparative Example
1.
[Comparative Example 3]
[0086] A dispersion was prepared in the same manner as in Example 14 except that ultrapure
water was added instead of the metal ion-containing compound used in Example 14.
[Table 1]
|
Anion-modified cellulose |
Added compound |
Monovalent or divalent metal ion |
B-type viscosity (mPa•s) |
Conc. (%) of added metal ion (v.s. cellulose) |
Ex. 1 |
Carboxylated cellulose |
Sodium glucuronate |
Na |
4569 |
1.6 |
Ex. 2 |
Carboxylated cellulose |
Sodium cellouronate |
Na |
4020 |
1.7 |
Ex. 3 |
Carboxylated cellulose |
Sodium salt of carboxymethyl cellulose |
Na |
3654 |
1.4 |
Ex. 4 |
Carboxylated cellulose |
Sodium ethylenediaminetetraacetate |
Na |
3695 |
0.9 |
Ex. 5 |
Carboxylated cellulose |
Sodium phytate |
Na |
3667 |
0.4 |
Ex. 6 |
Carboxylated cellulose |
Sodium polyacrylate |
Na |
3279 |
3.7 |
Ex. 7 |
Carboxylated cellulose |
Sodium polysulfonate |
Na |
3459 |
1.7 |
Ex. 8 |
Carboxylated cellulose |
Sodium polystyrene sulfonate |
Na |
4329 |
1.9 |
Ex. 9 |
Carboxylated cellulose |
Sodium salt of β-sulfated cyclodextrin |
Na |
4068 |
1.2 |
Ex. 10 |
Carboxylated cellulose |
Sodium chloride |
Na |
2407 |
5.9 |
Ex. 11 |
Carboxylated cellulose |
Mixture of sodium polyacrylate and magnesium polyacrylate |
Na, Mg |
4570 |
3.8 |
Ex. 12 |
Carboxylated cellulose |
Magnesium chloride |
Mg |
6128 |
3.8 |
Ex. 13 |
Carboxylated cellulose |
Potassium chloride |
K |
3569 |
7.9 |
Ex. 14 |
Carboxymethylated cellulose |
Sodium chloride |
Na |
2959 |
5.9 |
Comp. Ex. 1 |
Carboxylated cellulose |
(Ultrapure water) |
— |
7068 |
— |
Comp. Ex. 2 |
Carboxylated cellulose |
Polyvinyl alcohol |
— |
7728 |
— |
Comp. Ex. 3 |
Carboxymethylated cellulose |
(Ultrapure water) |
— |
4999 |
— |
[Example 15]
(Preparation of bleached LDKP)
[0087] Into a 2.4 L-volume rotary autoclave, 300 g (bone dry weight) of hardwood chip was
put, and water was added to adjust the liquor ratio to 2 L/kg. The mixture was maintained
at 170°C for 30 minutes to carry out hydrolysis treatment and then neutralized with
a neutralizing solution at 155°C for 15 minutes. The neutralizing solution was prepared
by mixing sodium hydroxide and sodium sulfide so that the solution had 11% active
alkali (versus the mass of chip), 25% sulfidity and a liquor ratio of 2.5 L/kg. After
the neutralization treatment, liquid was withdrawn from the autoclave, a cooking liquor
(which was prepared by mixing sodium hydroxide and sodium sulfide so that the cooking
liquor had 9% active alkali (versus the mass of chip), 25% sulfidity and a liquor
ratio of 2.5 L/kg) was added, and a cooking process was carried out at 160°C until
the H-factor reached 830.
[0088] The cooked unbleached pulp was delignified with oxygen and then bleached in the sequence
of DO-E/P-D 1 as ECF bleaching. The oxygen delignification was performed with Quantum
high intensity mini mixer, and after the reaction, the pulp was well washed. The bleaching
steps were all carried out in a water bath, using pulp slurry (pulp conc. 10%) contained
in a plastic bag. After the bleaching, fresh water was used for dilution to a pulp
concentration of 1.5% and water obtained by pressing was used for several-time washing.
In the subsequent bleaching stage, the water obtained by pressing in the previous
stage was used to adjust the pulp concentration to 15%, and then bleaching was carried
out with a predetermined amount of a bleaching chemical to adjust the pulp concentration
to 10%. It is to be noted that waste water obtained in the previous oxygen delignification
stage was not introduced in only the DO stage. Oxygen delignification: pulp conc.
10%; additive amount of sodium hydroxide 4.0%; initial oxygen pressure 6.0 kg/cm
2; reaction temperature 98°C; reaction time 60 min. D0: pulp conc. 10%; additive amount
of chlorine dioxide 10 kg/ADTP; reaction temperature 55°C; reaction time 40 min. E/P:
pulp conc. 10%; additive amount of sodium hydroxide 7.0 kg/ADTP; additive amount of
hydrogen peroxide 2.7 g/ADTP; reaction temperature 65°C; reaction time 90 min. D1:
pulp conc. 10%; additive amount of chlorine dioxide 1.5 kg/ADTP; reaction temperature
65°C; reaction time 180 min. By the bleaching treatment described above, unbeaten
bleached pulp (bleached LDKP; brightness: 86%) was obtained.
(Anionic modification of LDKP)
[0089] 100 g (bone dry weight) of LDKP obtained by repeating the procedure described above
was added to 10 L of an aqueous solution in which 1.56 g of TEMPO (Sigma Aldrich)
and 15.1 g of sodium bromide were dissolved, and the mixture was stirred until the
pulp was evenly dispersed. To the reaction system, 360 ml of an aqueous sodium hypochlorite
solution (available chlorine: 5%) was added, and then the mixture was adjusted to
pH 10.3 with a 0.5N aqueous hydrochloric acid solution and oxidation reaction was
initiated. While the pH of the system decreased during the reaction, a 0.5N aqueous
sodium hydroxide solution was successively added to adjust the system to pH 10. After
2-hour reaction, the mixture was passed through a glass filter and pulp obtained was
well washed with water to give oxidized pulp.
(Alkaline hydrolysis of anion-modified cellulose)
[0090] To 30 g (bone dry weight) of the oxidized pulp, 11.25 ml of 1 mol/L NaOH and 4 ml
of a 30% hydrogen peroxide solution were added, and the mixture was adjusted with
ultrapure water to 5% (w/v) and then heated in an autoclave at 80°C for 2 hours.
(Recovery of waste liquor obtained from alkaline hydrolysis reaction)
[0091] The resulting alkaline hydrolysate of the oxidized pulp was subjected to solid-liquid
separation by centrifugation.
(Defibration of anion-modified cellulose)
[0092] The solid content obtained by the above-mentioned solid-liquid separation was washed
and dewatered, then adjusted with sulfuric acid to pH 2, further dewatered by centrifugation,
and neutralized with sodium hydroxide to prepare concentrated oxidized pulp having
a concentration of 10% (w/v) and pH 7. The concentrated oxidized pulp was treated
10 times with an ultrahigh pressure homogenizer to give a transparent gelatinous dispersion
(10% (w/v)).
(Addition of waste liquor obtained from alkaline hydrolysis reaction)
[0093] To this gelatinous dispersion, the solution obtained by solid-liquid separation of
the alkaline hydrolysate was added to adjust the concentration to 6.7% (w/v), and
the mixture was stirred to homogenize it.
[0094] The B-type viscosity (60 rpm, 20°C) of the 6.7% (w/v) cellulose nanofiber dispersion
obtained was measured with TV-10 viscometer (Toki Sangyo Co., Ltd.).
[Example 16]
(Anionic modification)
[0095] 100 g (bone dry weight) of unbeaten bleached softwood pulp (Nippon Paper Chemicals
Co., Ltd.; brightness: 86%) was added to 10 L of an aqueous solution in which 1.56
g of TEMPO (Sigma Aldrich) and 15.1 g of sodium bromide were dissolved, and the mixture
was stirred until the pulp was evenly dispersed. To the reaction system, 360 ml of
an aqueous sodium hypochlorite solution (available chlorine: 5%) was added, and then
the mixture was adjusted to pH 10.3 with a 0.5N aqueous hydrochloric acid solution
and oxidation reaction was initiated. While the pH of the system decreased during
the reaction, a 0.5N aqueous sodium hydroxide solution was successively added to adjust
the system to pH 10. After 2-hour reaction, the mixture was passed through a glass
filter and pulp obtained was well washed with water to give oxidized pulp.
(Alkaline hydrolysis and recovery of waste liquor)
[0096] To 30 g (bone dry weight) of the oxidized pulp, 11.25 ml of 1 mol/L NaOH and 4 ml
of a 30% hydrogen peroxide solution were added, and the mixture was adjusted with
ultrapure water to 5% (w/v) and then heated in an autoclave at 80°C for 2 hours.
[0097] The resulting alkaline hydrolysate of the oxidized pulp was subjected to solid-liquid
separation by centrifugation.
(Defibration and addition of waste liquor obtained from alkaline hydrolysis)
[0098] The solid content obtained by the above-mentioned solid-liquid separation was washed
and dewatered, then adjusted with ultrapure water to 3% (w/v) and treated 10 times
with an ultrahigh pressure homogenizer to give a transparent gelatinous dispersion
(3% (w/v)).
[0099] To this gelatinous dispersion, the solution obtained by solid-liquid separation of
the alkaline hydrolysate was added to adjust the concentration to 2% (w/v), and the
mixture was stirred to homogenize it. The B-type viscosity (60 rpm, 20°C) of the 2%
(w/v) cellulose nanofiber dispersion obtained was measured with TV-10 viscometer (Toki
Sangyo Co., Ltd.).
[Example 17]
[0100] The oxidized pulp that was obtained in the same manner as in Example 15 was adjusted
with ultrapure water to 4% (w/v) and then treated 10 times with an ultrahigh pressure
homogenizer to give a transparent gelatinous dispersion (4% (w/v)).
[0101] To this gelatinous dispersion, the solution obtained by solid-liquid separation of
the alkaline hydrolysate in Example 15 was added to adjust the concentration to 3%
(w/v), and the mixture was stirred to homogenize it. The B-type viscosity (60 rpm,
20°C) of the 3% (w/v) cellulose nanofiber dispersion obtained was measured with TV-10
viscometer (Toki Sangyo Co., Ltd.).
[Comparative Example 3]
[0102] To the transparent gelatinous dispersion (10% (w/v)) obtained in the same manner
as in Example 15, ultrapure water was added to adjust the concentration to 6.7% (w/v),
and the mixture was stirred to homogenize it. The B-type viscosity (60 rpm, 20°C)
of the 6.7% (w/v) cellulose nanofiber dispersion obtained was measured with TV-10
viscometer (Toki Sangyo Co., Ltd.).
[Comparative Example 4]
[0103] To the transparent gelatinous dispersion (3% (w/v)) obtained in the same manner as
in Example 16, ultrapure water was added to adjust the concentration to 2% (w/v),
and the mixture was stirred to homogenize it. The B-type viscosity (60 rpm, 20°C)
of the 2% (w/v) cellulose nanofiber dispersion obtained was measured with TV-10 viscometer
(Toki Sangyo Co., Ltd.).
[Comparative Example 5]
[0104] To the transparent gelatinous dispersion (4% (w/v)) obtained in the same manner as
in Example 17, ultrapure water was added to adjust the concentration to 3% (w/v),
and the mixture was stirred to homogenize it. The B-type viscosity (60 rpm, 20°C)
of the 3% (w/v) cellulose nanofiber dispersion obtained was measured with TV-10 viscometer
(Toki Sangyo Co., Ltd.).
[Table 2]
|
Raw material pulp |
Alkaline hydrolysis |
Diluent |
Conc. (%) of CNF |
B-type viscosity (mPa•s) |
Ex. 15 |
Hardwood DKP |
done |
Solution from alkaline hydrolysis |
6.7 |
3210 |
Ex. 16 |
Softwood KP |
done |
Solution from alkaline hydrolysis |
2 |
1090 |
Ex. 17 |
Hardwood DKP |
- |
Solution from alkaline hydrolysis |
3 |
3460 |
Comp. Ex. 3 |
Hardwood DKP |
done |
Water |
6.7 |
4675 |
Comp. Ex. 4 |
Softwood KP |
done |
Water |
2 |
1970 |
Comp. Ex. 5 |
Hardwood DKP |
- |
Water |
3 |
6430 |
[Example 18]
[0105] Bleached LDKP (brightness: 86%) was prepared in the same manner as in Example 15
and subjected to anionic modification (introduction of carboxyl groups) to prepare
oxidized pulp. This oxidized pulp was subjected to alkaline hydrolysis.
[0106] The unwashed alkaline hydrolysate of the oxidized pulp was treated three times with
an ultrahigh pressure homogenizer (treatment pressure: 140 MPa) to give a transparent
gelatinous dispersion.
[0107] The B-type viscosity (60 rpm, 20°C) of the 5% (w/v) cellulose nanofiber dispersion
obtained was measured with TV-10 viscometer (Toki Sangyo Co., Ltd.).
[Comparative Example 6]
[0108] The alkaline hydrolysate of oxidized pulp which was obtained in the same manner as
in Example 18 was subjected to solid-liquid separation by centrifugation. The solid
content obtained was washed and dewatered, then adjusted with ultrapure water to a
concentration of 5% (w/v), and treated three times with an ultrahigh pressure homogenizer
(treatment pressure: 140 MPa) to give a transparent gelatinous dispersion.
[0109] The B-type viscosity (60 rpm, 20°C) of the 5% (w/v) cellulose nanofiber dispersion
obtained was measured with TV-10 viscometer (Toki Sangyo Co., Ltd.).
[Table 3]
|
Alkaline hydrolysis |
Washing |
No. of treatment with ultrahigh pressure homogenizer |
Conc. (%) of CNF |
B-type viscosity (mPa•s) |
Ex. 18 |
done |
- |
3 |
5 |
689 |
Comp. Ex. 6 |
done |
done |
3 |
5 |
6015 |